A variety of electric devices have been employed recently. As a kind of the electric devices, lithium-ion secondary batteries have been known. Since the lithium-ion secondary batteries have small sizes and large capacities, they have been used widely as a secondary battery for cellular phones, notebook-size personal computers, and so on. In recent years, their use as a battery for electric automobiles, hybrid automobiles, and so forth, has also been proposed.
Lithium-ion secondary batteries possess an active material, which can insert and eliminate lithium (Li), in the positive electrode and negative electrode, respectively. Lithium-ion secondary batteries operate by means of the movements of lithium ions between the two electrodes.
As a negative-electrode active material for lithium-ion secondary battery, carbon materials having a multilayer structure have been used mainly. Using this sort of carbon material leads to making it possible to inhibit the decline of discharge capacity after being charged and discharged repetitively, thereby making the cyclability of lithium-ion secondary batteries upgradable. However, lithium-ion secondary batteries, whose negative-electrode active material is constituted of these carbon materials alone, are associated with such a problem that they are poor in terms of the initial capacity (or energy density).
In order to enhance the initial capacity of lithium-ion secondary batteries, it has been proposed to use elements, which can be alloyed with Li and whose theoretical capacity is larger than that of carbon materials, as a negative-electrode active material. Since silicon (Si), an element being capable of alloying with Li, has a larger theoretical capacity than do carbon materials and the other elements (e.g., tin and germanium), it has been believed that Si is useful as a negative-electrode active material for use in lithium-ion secondary batteries. That is, compared with using carbon materials, using Si as a negative-electrode active material makes it possible to obtain higher-capacity lithium-ion secondary batteries.
In the meanwhile, Si undergoes volumetric changes greatly, being accompanied by the occlusion and release (or sorption and desorption), and vice versa, of Li at the time of charging and discharging. The volumetric changes turn Si into fine particles to fall down or come off from current collectors, so that there is such a problematic issue that the charging-discharging cyclic longevity of the resulting batteries is shorter. Hence, compared with the case where Si is used as a negative-electrode active material, using silicon oxide as a negative-electrode active material results in making it more possible to inhibit the volumetric changes that are accompanied by the sorption and desorption, and vice versa, of Li at the time of charging and discharging.
For example, it has been investigated to employ a silicon oxide (i.e., SiOx where “x” is 0.5≦“x”≦1.5 approximately) as a negative-electrode active material. “SiOx” is a general formula expressing the generic term for amorphous silicon oxide that is obtainable from metallic silicon (Si) and silicon dioxide (SiO2) serving as the raw materials. It has been known that SiOx decomposes into silicon (Si) and silicon dioxide (SiO2) when being heat treated. This is referred to as a “disproportionation reaction.” When the silicon oxide is homogeneous solid silicon monoxide (SiO) in which the ratio between Si and O is 1:1 roughly, the silicon oxide separates into two phases, a silicon (Si) phase and a silicon dioxide (SiO2) phase, by means of the internal reactions in the solid. The Si phase, which is obtainable as an outcome of the separation is fine extremely, so that it is dispersed within the SiO2 phase. Moreover, the SiO2 phase covering the Si phase possesses an action of inhibiting electrolytic solutions from decomposing. Therefore, lithium-ion secondary batteries, in which a negative-electrode active material comprising SiOx that has been decomposed into Si and SiO2 is used, excel in the cyclability.
Incidentally, SiOx is poor in terms of the conductivity comparatively. Consequently, negative electrodes including SiOx as the negative-electrode active material are also poor in terms of the conductivity. Therefore, it has been desired to upgrade the conductivity of the negative electrodes including SiOx. In order to upgrade the conductivity of the negative electrodes, it is believed to be good to blend a material excelling in the conductivity, namely, a conductive additive, into negative electrodes. Moreover, the particle diameter of conductive additive is usually smaller than the particle diameter of SiOx. Consequently, the resulting negative-electrode active material's surface can be covered with a conductive additive by blending the conductive additive more into the negative-electrode active material. Because conductive additives have a function of retaining electrolytic solutions as well, the electrolytic solutions become widespread fully at around the surface of the negative-electrode active material. Consequently, it is believed that the discharge capacity of the resultant lithium-ion secondary batteries upgrades.
Since KETJENBLACK (or KB) having been used generally as a conductive additive is carbonaceous fine particles, it excels in the conductivity. In the meantime, however, because KB has a hollow configuration so that the specific surface area is large, it has such a problem that it is likely to agglomerate. When a conductive additive has been agglomerated, it is less likely to disperse the conductive additive uniformly within the resulting negative-electrode active material, so that it is less likely to upgrade the resultant conductivity fully. It is believed that, when using a conductive additive whose specific surface area is smaller than that of KB, it is believed possible to circumvent the drawback resulting from the agglomeration of the conductive additive.
For example, in Patent Literature No. 1, a technique is disclosed in which acetylene black (or AB) serving as a conductive additive is blended into a negative-electrode active material for lithium-ion secondary battery in which SiOx serves as a negative-electrode active material. However, blending AB simply into a negative-electrode active material might possibly give rise to a case where it is less likely to upgrade the battery characteristics of the resulting lithium-ion secondary battery. For example, when a conductive additive has been blended into a negative-electrode active material excessively in order to intend to upgrade the resultant conductivity, the rate of adhesion between the conductive additive and SiOx has declined because the surface area of the conductive additive has become enormous, so that there might possibly arise such a case where the resulting discharge capacity declines. Moreover, in such an instance, the amount of SiOx has declined with respect to that of the conductive additive. On account of this as well, there might possibly arise such a case where the resultant discharge capacity declines. Thus, current electric storage devices being represented by lithium-ion secondary batteries have not yet arrived at satisfying a variety of characteristics to be required. Therefore, it has been desired to develop electric storage devices that excel in various characteristics (being hereinafter called “battery characteristics”) as an electric storage device.